1. Field of the Invention
The present invention relates generally to a system and method for controlling time-division multiplex communications between a master station and a number of slave stations. A polling method is adopted in which a response is returned from one slave station to a master station in response to a que issued from the master station to the slave station in a communication system established between the master station and a plurality of slave stations connected via a communication circuit, and which are applicable to a multiple network, e.g., installed in an automotive vehicle. More particularly, the present invention relates to a system and method for controlling the multiplex communication which supervises a state of the communication and the master station at one of the slave stations.
2. Description of the background art
Various types of multiplex communication control systems have been proposed.
For example, one of the previously proposed communication control systems includes a master station and a plurality of slave stations, the master station being connected to the slave stations via a serial bus which serves as a communication circuit so that the master station can communicate with each slave station. Specifically, the master station requests one of the slave stations to transmit data input into the requested slave station to the master station in response to a request from the master station. Thereafter, the master station processes the output data for transmission to one of the slave stations selected by the master station.
If the master station fails, or if an abnormality occurs such as a breakage in the serial bus, the slave station detects the abnormality in the master station and/or in the serial bus and, in a case where the output data of the slave station controls is used to control equipment, it is necessary to activate a fail-safe device so as not erroneously operate the equipment.
The master station includes a transmission/reception circuit to communicate with each slave station and a microcomputer for controlling the transmission/reception operations to or from each slave station via the transmission/reception circuit. Each slave station includes a transmission/reception circuit for communicating with the master station and a master station monitoring circuit for monitoring the state of the master station and the serial bus. The circuit for master station monitoring includes a signal detection circuit for detecting a communication signal always transmitted from the master station when the master station operates normally, a counter resettable by means of an output signal derived from the signal detection circuit, and a register including, e.g., a flip-flop circuit (FF) which supplies a low-level output signal to the master station monitoring circuit as the master station monitor error signal.
An operation of the previously proposed multiplex communication control system will be described below.
If the master station operates normally, a communication signal is transmitted via the serial bus from a transmission/reception circuit under control of the microcomputer. In the slave stations, a signal detection circuit of the master station monitoring circuit detects the communication signal and outputs a detection signal representing a change point of the communication signal detected by the signal detection circuit. A detection signal output from the signal detection circuit is supplied to a counter and register. The detection signal is used to reset the counter and to set the register. Consequently, if the master station operates normally and the communication signal is always transmitted, the counter is always reset in response to the detection signal derived from the signal detection circuit. Since a carry signal is not output from the counter, the register is not reset. Since the register is continually reset in response to the detection signal from the signal detection circuit, the register does not output the low level master station monitoring error signal.
On the other hand, suppose that the master station fails or the serial bus operates in an abnormal state. In this case, the signal detection circuit of the master station monitoring circuit cannot detect the communication signal. Therefore, the register is not set and the counter is not reset. Consequently, the counter outputs a carry signal after the counting operation is ended for a predetermined period of time so that the register is reset. Then, the register outputs the low level master station monitoring error signal. The error signal is supplied to the transmission/reception circuit. Consequently, the transmission/reception circuit fixes the output data in a safety mode. It is noted that the predetermined period of time during which the counter counts is selected to be several times a normal communication frame period.
If the problem is corrected and communication signal transmission is resumed from the master station, the communication signal is detected at the slave station by means of the signal detection circuit of the master station monitoring circuit provided for each slave station. The detection signal is used to reset the counter. The register, therefore, is set. Since the master station monitoring error signal is eliminated from the register, the respective slave stations can recognize that communication has returned to its normal state.
However, a problem arises in the previously proposed communication control system.
That is to say, since the master station monitoring circuit of each slave station always monitors a state of the serial bus, it is necessary for the master station to always transmit the communication signal to the serial bus. Consequently, since it is necessary for the microcomputer of the master station to activate the transmission/reception circuit of the master station at constant intervals, the microcomputer is compelled to carry out interrupt processing at those intervals. Therefore, the microcomputer receives a burden in view of its software in a case where another task is being executed and communication control and slave station control must also be carried out. Consequently, a time required for the microcomputer to carry out a given task becomes longer.